Congratulations on making your event so successful It's unfotunate that some unscrupulous people are trying to take advatage of your success by reselling tickets at high prices
I wish I could offer some brilliant suggestion that would prevent it, but probably anything I could think of you already have, or someone would find a way around....
Here are a couple of suggestions I thought of, though for next year. Maybe they would help limit toutes (is that really what you call them? We call them scalpers here, but I think it's the same idea)
1. Limit the number of tickets are person can buy in one transaction. I just checked in Eventbright and it would let me order 30 Friday tickets if I wanted to. What legimate person needs 30 tickets? I would say limit it to two or three. This may not stop the toutes, but would at least slow them down. Yet it would still let couples or small groups of friends order together. Also if you noticed you had 10 orders of three tickets come from the same address, it would be suspicious and could be investiagted....
2. Right now when you buy a ticket you put in the Name of the purchaser. My suggestions is to also have a box where a person can put "Name on Ticket" and make it a required field, one for each ticket bought. For many people who are buying tickets for themseles it would merely mean typing their name twice (not a big deal - although I'm sure some would grouse...) It would, however, help alleviate the panicked messages we are seeing about "I bought the ticket for my spouse, child, bff, bae, etc. can they still use it if I'm not with them?". When a person buys the tickets they can put those people's names on them. That way if there is a problem you can check the ticket name against I.D.. If someone buys a ticket off of eBay, then they are running the risk that they will get asked for I.D. and it won't match the name on the ticket.
3. The worst people are those who are going to sell illegimate tickets. I can see some bad people buying one ticket and then selling it multiple times. There is nothing really you can do to prevent that and it's not your fault, but you can help prevent some people getting taken in by giving them lots of warning. When the tickets show as "Sold Out" on your website put a disclaimer underneath that tickets are not-transferable; tickets purchased from secondary sources may not be honoured at the event. You could also add a line similar to that to your tickets, or maybe include something about "You may be required to show photo I.D. that matches the name on this ticket or be denied entry". Like I said, this probably won't stop the unscrupulous, but at least it will hopefully stop some people from making foolish purchases on eBay (yes I know that some people will still do it anyway no matter how often warned, but when they do it also gives you more ammunition for when you have to tell them, sorry - this ticket has already been scanned through you cannot use it). This way if I bought a photo ticket for my best friend Fred and now suddenly he can't go, I could go with my new best friend Joe. Yes, his name won't match the name on the ticket, but since we're not unscrupulous people and are only using the ticket once, it won't even be noticed. My neighbour, though, who foolishy bought his ticket from a scalper on eBay, however, has to hope that the scalper was honest and didn't sell the ticket to five other people since it would come back as an error and his ID would not match the ticket (so he would be SOL).
4. As you suggested you would do, offer a buy back program so honest people have a way to return their tickets, get their money back and let other people legitmately have them. However, as you also mentioned the time right before the show is your busiest time. Therefore I suggest having a cut-off date maybe two or three weeks before the show when you can no longer return your tickets. Yes I know some people won't be happy because life happens and things could happen right before the show, but there needs to come a point when we, the customer, need to recognize that there are limits to how much we can expect. This might take some pressure off the sell outs as there wouldn't be any "wasted" tickets (ie tickets bought but then not used), or at less there would be fewer. (Although I suspect the numbers of tickets bought that now a person realizes they can't use would actually be quite low, since up to this year it has been all sales are final.)
5. Continue to work with eBay and similar reselling sites. Tell them that tickets are not transferable (and would not be honoured) and ask them to take the auctions or listings down. Mention on your rules and regulations that tickets are not resellable and any tickets that are resold will be null and void. And if anyone is foolish enough to post a picture that shows the barcode or some other identifying information about the ticket - cancel it in your system.
6. Post lots of warnings about fake tickets and scams around your entry so people will know they will not be honoured. Yes I'm sure someone will find a way to make fake tickets, even with their own names on them, so when these get scanned and you get an error reading you can point to the warnings about fake tickets and remind them caveat emptor.
Anyway, I don't know if any of those ideas are of any use to you or not, but they were all I could think of. Again, congratulations on how popular you have made your show. It looks like it will be a fantastic one! I can't wait to see it next week.